I needed some for a kindergaten science experiment a couple of weeks ago. The big one in Itaewon had it in the back somewhere but couldn't find it whilst I waited. They told me to come back the next day but I ended up going to the other store and they had Disprin which they claimed to be the same...

I do understand the appeal of a particular item that you're familiar with, but just in case you don't know, Alka Seltzer is basically sodium bicarbonate and aspirin with a touch of citric acid--I'm pretty sure that you could find it (or a different brand equivalent) in pretty much any drugstore.

Or you could always just make the homemade variety like people used to do: mix sodium bicarbonate (baking soda) with water, add a squeeze of lemon (or orange--go crazy with it!) and take an aspirin.